Semiconductor packages are electronic devices for accommodating active components such as semiconductor chips, whose structure is primarily composed of a chip mounted on a substrate and electrically connected to the substrate via conductive elements such as bonding wires, and an encapsulation body formed by a resin compound (such as epoxy resin, etc.) on the substrate to encapsulate and protect the chip and bonding wires against external moisture and contaminant. The resin compound forming the encapsulation body is usually opaque or non-transparent, thereby making a photosensitive chip such as CMOS (complementary metal oxide semiconductor) chip that requires light for operation not suitably incorporated in such a semiconductor package.
Accordingly, U.S. Pat. No. 6,060,340 provides a semiconductor package with a structurally modified encapsulation body for allowing light to reach the photosensitive chip. As shown in the semiconductor package of FIG. 5, a pre-fabricated encapsulation dam 13 is attached to a substrate 11 via an adhesive 16 and shaped as a wall structure to form a space 14 for receiving the photosensitive chip 10 and a plurality of bonding wires 12 that electrically connect the chip 10 to the substrate 11. A lid 15 is mounted on the encapsulation dam 13 to seal the space 14 and hermetically isolate the chip 10 and bonding wires 12 from the external atmosphere. The lid 15 is made of a light-permeable or transparent material to allow light to go through the lid 15 and reach the chip 10 to facilitate operation of the chip 10. However, the above adhesive has relatively higher moisture absorptivity; when the adhesive with the absorbed moisture is subject to a high temperature in subsequent fabrication processes, it would lead to popcorn effect and delamination between the substrate and the encapsulation dam, thereby damaging the reliability of the semiconductor package.
Another type of semiconductor package suitable for a photosensitive chip is disclosed by U.S. Pat. Nos. 6,262,479 and 6,590,269, whose fabrication steps are shown in FIGS. 6A and 6B and which does not require the above adhesive. Referring to FIG. 6A, a molding process is carried out to form the encapsulation dam 13 on the substrate 11. During molding, an encapsulation mold is used comprising an upper mold 17 having an upwardly-recessed cavity 170 with an insert portion 171 formed in the cavity 170, and a lower mold 18; thus this encapsulation mold is customarily named “insert mold” hereinafter. The substrate 11 is clamped between the upper and lower molds 17, 18, with the insert portion 171 coming into contact with the substrate 11 so as to cover the predetermined area for chip-attachment and wire-bonding on the substrate 11. A resin compound (such as epoxy resin) is injected to the upwardly-recessed cavity 170 to form the encapsulation dam 13 on the substrate 11. By provision of the insert portion 171, the predetermined area for chip-attachment and wire-bonding on the substrate 11 would not be encapsulated by the encapsulation dam 13 and is exposed after the upper and lower molds 17, 18 are removed from the substrate 11, as shown in FIG. 6B. Then, the photosensitive chip 10 and bonding wires 12 are readily mounted on the exposed area of the substrate 11. And finally with the lid 15 attached to the encapsulation dam 13, the semiconductor package is fabricated.
However, the above semiconductor package may still suffer significant drawbacks. The insert portion of the upper mold covers the area on the substrate not to be encapsulated by the resin compound during molding. However, it is not easy to properly control the clamping force between the insert portion and the substrate. If the insert portion is not sufficiently clamped on the substrate, flash of the resin compound may occur between the insert portion and the substrate and thus contaminates the area on the substrate predetermined for chip-attachment and wire-bonding. If the insert portion is too strongly pressed on the substrate, the substrate may be structurally damaged by the insert portion. Moreover, the insert mold is cost-ineffective to fabricate, which requires formation of the insert portion according to the size of substrate or predetermined area on the substrate to be covered by the insert portion. In other words, if the size of substrate or area on the substrate to be covered changes, a new mold with correspondingly-sized insert portion is required, which would undesirably increase the fabrication cost and complicate fabrication processes for the semiconductor package.
Therefore, the problem to be solved herein is to provide a method for fabricating a semiconductor package with a photosensitive chip, which can overcome the above drawbacks to prevent the occurrence of resin flash over the substrate and reduce the fabrication cost.